RAID (Redundant Array of Independent Disks) is one representative technology to improve reliability of data managed by a computer system. RAID allows the user to manage a combination of multiple hard disks as one redundant logical volume. There are multiple levels of RAID which differ in the data placement schemes and the data redundancy methods. For example, RAID 1, or mirroring, is the technique of writing the same data to more than one disk, and RAID 5 generates parity data from data stored on multiple disks and reconstructs lost data using the parity data.
Even if data management is done by storing data in a redundant manner (i.e., the same data is stored in more than one location), data redundancy may be lost due to a disk failure or the like. In such a case, the lost redundancy is restored using remaining data. The process of restoring data redundancy is referred to as a “rebuild process”. One proposed technology for the rebuild process is directed to a disk sharing method for a flexible magnetic disk device using a hot spare disk. According to this method, in the event an on-line magnetic disk device in a logical volume fails, a rebuild function is implemented to restore data on the failed magnetic disk device using data on the remaining magnetic disk devices in the same logical volume.    Japanese Laid-open Patent Publication No. 2005-099995
A RAID controller is capable of controlling multiple RAID groups. In the event a disk belonging to one of the controlled RAID groups fails, a rebuild process is executed for the RAID group. However, executing a rebuild process for one of the multiple RAID groups under the control of the RAID controller increases the load of the RAID controller, which adversely affects other controlled RAID groups of the RAID controller. For example, accesses to normally operating RAID groups needing no rebuild process are inhibited, causing access delays. Such problems occur not only in RAID groups but also in an entire system that implements data management by storing data redundantly and executes a process of restoring data redundancy after being lost.